Export BCCON/BCPROP for sector model

[kriben]

Suggested first implementation:

1. Create python API for exporting visible cell from a view to List[int] (1 for visible, 0 for invisible).
2. Create a grid result "BCCON" which has regions of different boundary conditions. 0 for undefined regions, 1 for first BC and so on. Use python API from point 1. to combine results from multiple views: visible cells in first view is set to BCCON=1, and so one. Default value is 0.
3. When exporting sector model: add a BCCON property to Deck file with one BCCON per cell face for each cells where BCCON grid property is different from 0. Expected syntax for cell [1, 2, 3]:
   BCCON
   1 1 1 2 2 3 3 X- /
   2 1 1 2 2 3 3 X /
   3 1 1 2 2 3 3 Y- /
   4 1 1 2 2 3 3 Y /
   5 1 1 2 2 3 3 Z- /
   6 1 1 2 2 3 3 Z /
   /